Owing to the rapid industrial and commercial development of urban areas in recent history, there is commonly inadequate space in cities for new construction. In order to maximize basement space, a hollow type of deep raft foundation is usually employed in new buildings. The depth to which the basement area is dug ranges from GL-12.0 meters to GL-30.00 meters, and may even exceed that depth. However, as the water table in a city is generally higher than in a rural area, if the area of a structure projecting above the ground is not significantly greater than the area of a foundation which has been dug, the weight of the structure may not be sufficient to stabilize the floating force produced by the underground water level. Such a structural design may cause the following problems to the raft foundation:
(1) A part of the raft foundation may be controlled by an uplift floating force (the weight of the structural body is lighter than the floating force produced). This will cause the bottom part of the foundation to be more easily separated from the soil, causing the foundation to "float" upward. This uplifting force may cause a break in the foundation from the large moment and shear to which the foundation would be subjected from such motion.
(2) The part of raft foundation which is being affected by the floating force may be displaced upward, while the remainder of the foundation may start to "sink", again creating the problem of breakage in the foundation.